1. Field of the Invention
The field of the invention is that of sensors for detecting the position of an object in two axes, such as a linear axis and a rotary axis.
2. Background Art
Sensors and devices for detecting the position of an object in a single axis are well known. For instance, a linear variable differential transformer (LVDT) can accurately detect the position of an object along a linear axis. In a similar manner, a rotary encoder can quickly and accurately detect the rotational position of a rotating device, such as a motor shaft. In another example, a plurality of hall-effect sensors may be used with a magnet to determine the rotary position of a nearby rotating shaft or wheel, as disclosed in U.S. Pat. No. 4,737,710.
Instead of hall-effect sensors, magnetoresistive elements may be used, as outlined in U.S. Pat. No. 5,055,781. A magnetoresistive element is an element whose electrical resistance varies in a predictable way when subjected to a magnetic field. Examples are ferromagnetic alloys, such as Ni—Co on a substrate. Additional examples are given in U.S. Pat. No. 5,861,747, which discloses multi-layer magnetoresistive elements, such as multilayered Cu/Co.
However, considerable difficulties emerge if it is desired to detect the position of an object in more than one dimension or axis. For instance, superconducting quantum interference devices (SQUIDs) may be used to detect magnetic fields or their movement in more than one axis by using more than one SQUID, as outlined in U.S. Pat. No. 4,489,274. A disadvantage of such detectors is that they require cryogenic temperatures down to 4 K, which severely restricts applications for which such detectors are used.
In the field of toroidal-type continuously-variable transmissions, such as for automotive use or for an on- or off-road vehicle, the axial and rotational position of trunnions of the transmission determines the ratio of input speed to output speed, as shown in U.S. Pat. No. 5,885,185. It is known to determine the position of the trunnions by using a control mechanism with a stepping motor and a sleeve and spool that drives the trunnions and associated rollers to their desired positions. The linear position of the trunnions may be estimated by a linear-axis displacement unit. Alternatively, the position of the trunnions may be estimated by using a linear axis displacement sensor. Angular position may be determined by a rotation sensor. While this apparatus can determine approximate positions, it would be desirable to provide an improved method and apparatus for detecting the axial and rotational positions of trunnions with greater accuracy and in a single package.